1. Field of the Invention
The present invention relates to a sensor utilizing attenuated total reflection (hereinafter referred to as ATR), such as a surface plasmon resonance sensor that detects the refractive index of a sample by utilizing the generation of surface plasmon, and more particularly to a sensor, utilizing ATR, which detects the refractive index distribution of a sample and a sensor, utilizing ATR, which detects a two-dimensional measuring region with a parallel light beam.
2. Description of the Related Art
In metals, if free electrons are caused to vibrate in a group, compression waves called plasma waves will be generated. The compression waves generated in a metal surface are quantized and called surface plasmon.
A wide variety of surface plasmon resonance sensors have been proposed to quantitatively analyze a substance in a sample by taking advantage of a phenomenon that surface plasmon is excited by light waves. Among such sensors, one employing a system called “Kretschmann configuration” is particularly well known (e.g., see Japanese Unexamined Patent Publication No. 6(1994)-167443).
The surface plasmon resonance sensor employing the aforementioned system is equipped with a dielectric block formed, for example, into the shape of a prism; a metal film, formed on a face of the dielectric block, for placing a sample thereon; and a light source for emitting a light beam. The sensor is further equipped with an optical system for making the light beam enter the dielectric block so that a condition for total internal reflection is satisfied at the interface between the dielectric block and the metal film and that various angles of incidence, including a surface plasmon resonance condition, are obtained; and photodetection means for measuring the intensity of the light beam totally reflected at the interface, and detecting the state of surface plasmon resonance.
In the surface plasmon resonance sensor mentioned above, if a light beam strikes the metal film at a specific incidence angle θsp equal to or greater than a critical angle of incidence at which total internal reflection takes place, evanescent waves with an electric field distribution are generated in a sample in contact with the metal film, whereby a surface plasmon is excited at the interface between the metal film and the sample. When the wave vector of the evanescent light is equal to the wave number of the surface plasmon and therefore the wave numbers between the two are matched, the evanescent waves and the surface plasmon resonate and light energy is transferred to the surface plasmon, whereby the intensity of the light totally reflected at the interface between the dielectric block and the metal film drops sharply. The sharp intensity drop (i.e., ATR) is generally detected as a dark line by the above-mentioned photodetection means. The relationship between the incidence angle θ of a light beam with respect to the interface and the intensity I of the light beam totally reflected at the interface is shown in FIG. 2. In the figure, the specific incidence angle θsp indicates an incidence angle at which ATR occurs.
Note that the above-mentioned resonance occurs only when the incident light beam is a p-polarized light beam. Therefore, in order to make the resonance occur, it is necessary that a light beam be p-polarized before it strikes the interface.
If the wave number of the surface plasmon is found from the specific incidence angle θsp at which ATR takes place, the dielectric constant of a sample can be calculated by the following Equation:             K              S        ⁢                                  ⁢        P              ⁡          (      ω      )        =            ω      c        ⁢                                                      ɛ              m                        ⁡                          (              ω              )                                ⁢                      ɛ            s                                                              ɛ              m                        ⁡                          (              ω              )                                +                      ɛ            s                              where Ksp represents the wave number of the surface plasmon, ω represents the angular frequency of the surface plasmon, c represents the speed of light in vacuum, and εm and εs represent the dielectric constants of the metal and the sample, respectively.
If the dielectric constant εs of a sample is found, the density of a specific substance in the sample is found based on a predetermined calibration curve, etc. As a result, a specific substance in the sample can be quantitatively analyzed by finding the specific incidence angle θsp at which the intensity of the reflected light drops sharply.
In addition, a leaky mode sensor is known as a similar sensor making use of ATR, as disclosed, for instance, in “Spectral Research,” Vol. 47, No.1 (1998), pp. 21 to 23 and pp. 26 to 27. The leaky mode sensor is equipped with a dielectric block formed, for example, into the shape of a prism; a cladding layer formed on a face of the dielectric block; and an optical waveguide layer, formed on the cladding layer, for placing a sample thereon. The leaky mode sensor is further equipped with a light source for emitting a light beam; an optical system for making the light beam enter the dielectric block at various angles of incidence so that a condition for total internal reflection is satisfied at the interface between the dielectric block and the cladding layer and so that ATR occurs by the excitation of a waveguide mode in the optical waveguide layer; and a photodetection means for measuring the intensity of the light beam totally reflected at the interface between the dielectric block and the cladding layer, and detecting the excited state of the waveguide mode, that is, the state of ATR.
In the leaky mode sensor with the construction mentioned above, if a light beam strikes the cladding layer through the dielectric block at angles of incidence equal to or greater than an angle of incidence at which total internal reflection takes place, the light beam is transmitted through the cladding layer and then only light with a specific wave number, incident at a specific incidence angle, propagates through the optical waveguide layer in a waveguide mode. If the waveguide mode is excited in this manner, the greater part of the incident light is confined within the optical waveguide layer, and consequently, ATR occurs in which the intensity of light totally reflected at the above-mentioned interface drops sharply. Since the wave number of light propagating in the optical waveguide layer depends on the refractive index of a sample on the optical waveguide layer, the refractive index of the sample and the properties of the sample related to the refractive index thereof can be analyzed by finding the above-mentioned specific incidence angle θsp at which ATR takes place.
In addition, the above-mentioned surface plasmon resonance sensors or leaky mode sensors can be used to measure the refractive index distribution, within a plane along the aforementioned interface, of a sample. In the case of the surface plasmon resonance sensors, the relationship between the incidence angle of a light beam with respect to the interface and the intensity of the light beam totally reflected at the interface is shown in FIG. 2. The specific incidence angle θsp shown in FIG. 2 indicates an angle at which ATR occurs. The aforementioned relationship between the incidence angle and the light intensity will be shifted in the horizontal direction of FIG. 2, if the refractive index of a sample varies. Therefore, if a light beam strikes the aforementioned interface at an incidence angle near the specific incidence angle θsp, the intensity of the light beam totally reflected at the interface varies with the refractive index of a sample. Hence, if a parallel light beam with a relatively wide beam section is caused to strike the interface, and an image carried by the parallel light beam totally reflected at the interface (i.e., intensity distribution within the beam section) is detected, the refractive index distribution of a sample within a plane along the interface can be detected.
The foregoing description of the surface plasmon resonance sensors applies to the leaky mode sensors, because the leaky mode sensors differ from the surface plasmon resonance sensors only in that total internal reflection is attenuated by the excitation of a waveguide mode in the waveguide layer instead of being attenuated by surface plasmon resonance. Therefore, it is also possible to detect the refractive index distribution of a sample by employing the leaky mode sensors.
In analyzing physical properties by the aforementioned surface plasmon sensors or leaky mode sensors, there are cases where a plurality of samples need to be measured under the same condition, or cases where the two-dimensional physical property information of a sample is needed. In such cases, the aforementioned surface plasmon sensors or leaky mode sensors can be utilized.
For instance, a description will be given of how the two-dimensional physical properties of a sample are analyzed by the surface plasmon resonance sensors. The relationship between the incidence angle of a light beam with respect to an interface and the intensity of the light beam totally reflected at the interface, as previously stated, is shown in FIG. 2. The specific incidence angle θsp indicates an angle at which ATR occurs. This relationship will be horizontally shifted if the refractive index of a substance on a metal film varies. Therefore, if a light beam strikes a two-dimensional region on the interface at a predetermined incidence angle, a portion of the region where ATR occurs at the incidence angle, that is, a light component incident on a point on the interface where a specific substance is present on the metal film, is detected as a dark line. Hence, if parallel light with a relatively wide cross section is employed and the light intensity distribution of the cross section of the light beam totally reflected at the interface is detected, the distribution of specific substances within a plane along the interface can be measured. Since the intensity of the reflected light is reduced at angles above and below the predetermined incidence angle θsp, as shown in FIG. 2, the light intensity distribution of the cross section of the light beam, incident on the interface at predetermined angles and reflected, indicates the two-dimensional refractive distribution of a substance (sample) present on the metal film.
The foregoing description of the surface plasmon resonance sensors applies to the leaky mode sensors, because the leaky mode sensors differ from the surface plasmon resonance sensors only in that total internal reflection is attenuated by the excitation of a waveguide mode in the waveguide layer instead of being attenuated by surface plasmon resonance. Therefore, it is also possible to detect the two-dimensional physical properties of a sample by employing the leaky mode sensors.
However, in the conventional sensor utilizing ATR, which is constructed to detect the refractive index distribution of a sample in the aforementioned manner, there are cases where the image by totally reflected light is distorted and therefore the refractive index distribution cannot be accurately measured.
On the other hand, in the sensor utilizing ATR, in which a parallel light beam is caused to strike an interface to detect the light intensity distribution of the cross section of reflected light, there are cases where a laser light is employed and, because of this, the light intensity distribution of a light beam detected by photodetection means (two-dimensional sensor) cannot be accurately measured due to coherent noise caused by the laser light beam. Particularly, in the case of a charge-coupled device (CCD) sensor being employed as a two-dimensional image sensor, there are cases where multiple interferences due to coherent noise take place within a protective film usually provided on the light-receiving face of the CCD sensor and therefore interference stripes occur on an image plane. Thus, there is a great influence due to coherent noise.